The transmission lines you see along the highway may strike you as ugly eyesores or as proud monuments to modern civilization. But whatever your point of view, one thing is certain: every one of those lines is the result of a complex physical and economic calculation, including a number of forward projections on load and capacity. Plus, while some stand-up comedians are called high voltage, the real high voltage is in those lines. Why is the voltage high?

Under the second law of thermodynamics, whenever power is transferred, some of it is lost. In the case of transmission lines, losses take the form of heat emanating from the conductors, leakage of current across insulators, ionization of pathways in the air surrounding the conductors, or just current running to ground. Also, magnetic and electric fields are created around the conductors, which create inductance and capacitance in the lines. The short answer is that the higher the voltage, the smaller the losses.

Under Ohm’s Law, the voltage (V) is equal to the product of the current (I) and the resistance (R) in the circuit, or V = I*R. The power (P) is the amount of the energy absorbed in the circuit (i.e., delivered to a load such as a customer’s machinery), which is equal to the voltage (V) times the current (I), or P = V*I. The power lost (P_{L}) when current flows through a conductor is the product of the resistance (R) of the conductor through which the current flows and the square of that current. P_{L} = I^{2}*R. Thus, if you lower the current (I), you reduce the amount of power lost.

But if you lower the current, under P = V*I you would have to raise the voltage in order to keep the same amount of power going to the load; that is, the power must be constant, so under P = V * I, V has to increase if I is lowered.

The reverse is also true. If you raise the voltage, you can decrease the current and keep the power constant. So if we were to raise the voltage by a factor of 100, the current would be reduced to 1/100th of its former value.

Going back to our loss equation, since power lost is proportional to the square of the current (I^{2}*R), reducing current by a factor of 100 reduces losses by a factor of 10,000 (.01 * .01 = .0001).

And that’s why high voltage makes sense.